1. Field of the Invention
This invention relates to a method for data transmission in two-way communication via low-voltage systems that are linked to a higher order telecommunication network, and to an arrangement for implementing said method.
2. Background of the Prior Art
Utility companies have highly ramified power supply networks through which they are connected with their customers. This benefit has for a long time been utilized beyond mere energy supply, for example, for tone frequency remote control where data was transmitted in one-way communication systems though with the disadvantage that there was no feedback.
More recently, however, proposals were made to enable the use of the low-voltage systems of utility companies for two-way communication independent of other carriers. While one-way communication just allows the collection of data such as meter readings for electricity, gas, water, etc. or registration of measured values such as temperature, pressure, or alarms, two-way communication can be used to query switching states and to control complex technical facilities. In addition to regular data transmission, the low-voltage system that utility companies have and to which each household is connected can be used for plain ordinary telephone service. According to a known proposal of this kind, the power suppliers who use their low-voltage system for telecommunication have to provide, on the one hand, facilities that act as data filters to make sure that the data is received by its addressees only. On the other hand, devices are required at the network stations that transpose the data to a copper, cellular, or fiber network that connects the stations. It has been assumed as yet that about 100 to 200 households can be connected to one network station. In compliance with the European Celenec EN 50065-1 standard, a theoretically usable data rate of up to 70 kbps would be available for data communication in duplex mode on a dedicated frequency band up to 95 kHz.
A two-way communication system for data transmission between a central station and side stations and between substations and end user facilities is known from DE 195 04 587. Node controllers linked to the low-voltage network function as substations, and a large-area telecommunication network such as a cellular data network or a circuit-switched network, in particular, an optical fiber network, is used for data transmission between the central station and the substations. The node controllers associated with the distributed network transformers are equipped with standard modems that provide an interface between the low-voltage and the large-area telecommunication network whereas a modem with repeater function is provided as an intermediate station on the transmission path between the node controller and the end user facility; data transmission within the local low-voltage network is based on the spread spectrum method.
Data transmission in low-frequency networks uses the frequency range up to 148.5 kHz that is permitted in Europe. However, one setback is that transmission quality is restricted in this frequency range due to numerous interference signals and a high noise level, another setback is that the narrow-band transmission frequency band is limited with regard to the number of subscribers and the bit rate per subscriber.
It is therefore the problem of this invention to provide such a method and such an arrangement for using low-voltage systems that differs from conventional systems in that it combines a high data transfer rate with improved transmission quality, transmission security in ISDN quality, and real-time processing.
This problem is solved according to the invention by a data transmission method for two-way communication using a low-voltage system linked to a higher-order telecommunications network in that data transmission within the low-voltage system takes place at a high-frequency range of up to 30 MHz using band spreading of data signals and a transmitting level below the specified interference or noise voltage limit of line and radio disturbance characteristics, in that said band-spread data is given a direction coding to specify a logical direction within the low-voltage system using different sequences of a family of pseudo-random numbers to enable multiple-user operation, and in that a correlator placed at an attenuation-dependent distance identifies and regenerates the binary sequences of data with their user-specific spreading and direction-specific coding are identified by correlation using specified sequences at attenuation-dependent distances within the low-voltage system, then said data sequences are regenerated and assigned new direction codes for forwarding.
Alternatively, the process of sequence generation and additional directional coding can be carried out by controlled selection of sequences from various sequence families. Another sequence family is used as direction ID in each network area for band spreading of each user signal.
The limits for radio and line frequency interference are much lower at a higher frequency range, e. g. 10 MHz, than in the frequency range up to 148.5 kHz. But narrow-band interference caused by harmonic waves from other frequency ranges occurs at this range, too, and even the standard radio transmitters interfere with data transmission at this frequency range. On the other hand, the specified maximum output levels, which are very low, must not be exceeded. Furthermore, a signal output at a low level may drop below the noise level due to transmission loss that increases with growing distance and frequency, so that non-spread signal can no longer be received.
Due to its low output level and the high attenuation at this frequency range, the signal to be transmitted would drop below the noise level at a transmission loss of 50 to 70 dB/100 m but in buried cables it can be received below noise level and successfully regenerated at a distance of 100 m. A direction coding using code, time, or frequency multiplexing converts the physical separation which is impossible with data transfer in low-frequency systems into a logical separation, thus enabling duplex operation. Code multiplexing also ensures a multiple-user structure. As direct sequence band spreading is used where, instead of a single information symbol, a sequence of pseudo-random numbers is transmitted in the same time, the bandwidth required for transmission increases by a factor that corresponds to the sequence of pseudo-random numbers. In this way, narrow-band sources of interference and frequency-selective attenuation properties lose their influence on the transmission system.
The proposed method of data transmission at a high frequency range facilitates low-cost bidirectional data transmission in real time via the low-voltage systems of utility companies if buried cables are used. Transmission channels in ISDN quality with a data rate of 64 kbps can be provided, and the overall transmission capacity of the low-voltage line between connected users and the interface between low-voltage system and higher order telecommunication network is a minimum of 2 Mbps for each the forward and back channel with a bit error rate of 10xe2x88x926 over 100 m.
In a further development of the invention, a family of pseudo-random sequences such as Gold sequences is used for user-specific band spreading. To prevent mutual interference of users or their terminals, different families of pseudo-random sequences are used in the various network areas.
In an advantageous embodiment, the logical direction of the data stream is preset using code multiplexing, i. e. multiplying the data stream by Walsh sequences the length of which is shorter than that of the band spreading sequences. Alternatively, additional multiplication by Walsh sequences can be left out when specially selected pseudo-random sequences that undercut each other are used in different network areas. The benefit would be reduced signal processing requirements in real-time signal processing but increased channel management requirements.
According to another feature of the invention, the forward and backward directions can be divided to indicate a logical direction in a low-voltage system using time and/or frequency multiplexing; here, the band-spread signals are transmitted in the transmit and receive directions on separate frequency bands or time slots.
First, in the initializing phase prior to the actual data transfer, an initializing sequence plus user ID and a logon sequence are output, and a spread sequence is assigned to the user terminal by means of the user ID.
The arrangement of the invention for carrying out the method consists of a low-voltage system and integrated user terminals, local line distributor boxes and network stations as well as a higher order telecommunications network, with network interworking units being assigned to the network stations to link the low-voltage system and the higher order telecommunications network and with repeater units being placed in the low-voltage system, characterized in that the network interworking units, the repeater units placed at specific distances depending on the degree of attenuation, and the user terminals are designed for band-spreading the data signals at a transmit level below the specified interference or noise voltage limit of line and radio disturbance characteristics and for direction-coding the data signals, and that the repeater units, in addition, are designed for regenerating and direction-specific forwarding of the data stream.
A CDMA processor for spreading the data using its allocated spread sequence and adding the direction code, a modulator for modulating the signals onto a carrier frequency, a controllable amplifier to adjust the input level required at the receiving end for optimum correlator performance, and a physical coupler for feeding the spread and direction-coded data stream into the low-voltage system are assigned to the user terminal. The receiver structure consists of a controllable low-noise input amplifier, an IQ demodulator, an equalizer, preferably a rake receiver, and a CDMA processor for despreading the data signals. The unspread data signals are conditioned for transmission by a channel encoder/decoder in the base band, e. g., a convolution encoder and a Viterbi decoder. A data multiplexer/demultiplexer passes the data on to the voice and data interface that can be configured for any common interface type (e. g., S0, analog a/b, Ethernet). The user terminal has a device ID and an additional SIM (Subscriber Identity Module) allowing partially mobile use of the system. All components are controlled by a microprocessor and a centralized clock generator. The clock signal is synchronized using the data signal received. The transmit and receive signals are fed via a filter or a frequency separator into a physical coupler that also supplies power to the user terminal. In the event of a power failure, operation can be continued for a limited period of time.
The repeater units integrated into the low-voltage system at local line distributor boxes, lamp posts, or, optionally, in house connection boxes include the same functional groups as the user terminal but the functional groups of digital signal processing (equalizer, CDMA processor, channel encoder/decoder) and, optionally, parts of the clock generating unit are configured in accordance with the number of channels to be regenerated multiplied by the number of signaling directions. The error-corrected data signals are fed from the channel decoder into the next channel encoder either directly or via a switching matrix. A device ID is also implemented in the system-like in the user terminal. In addition, the repeater units are characterized in that a data memory is integrated into the system in which the current channel assignments and the associated direction codes and sequence to be used as well as other signal source and sink information are stored. This data memory is managed through the microprocessor.
The network interworking units include the same functional groups as a repeater unit but multiple functional groups of digital data processing (equalizer, CDMA processor, channel encoder/decoder) are configured in accordance with the number of transmission channels to the higher order telecommunication facility provided plus the number of synchronization channels required per low-voltage system. Furthermore, there are multiple gateways and front ends towards low voltage configured depending on the number of network areas to be supplied.
The decoded data signals are fed via a switching matrix into the transmission system that converts the signals on the telecommunication network side into e.g. n * 2 Mbps transmission systems for copper, optical fiber lines, or cellular radio connections, n being dependent on the demand and capacities available and as a rule representing a number between 1 and 3.
A microprocessor system assigns the channels in the network interworking unit by configuring the switching matrix and the CDMA processors. The network interworking unit also has a device ID and a data memory in which the data of all active connections consisting of routing information, channel assignment, signal quality during the connection, user terminal ID, services used, and the assigned transmission channel are stored. Optionally, data rate and protocol adjustment systems can be placed between the switching matrix and the transmission facility to the higher order telecommunication network which adjust potential current or future data formats for a data service to the system structure of the data transmission system on the telecommunications side.
According to yet another feature of the invention, additional high-frequency attenuation equipment can be provided, if required, in highly branched wiring areas of the low-voltage system such as local line distributor boxes, network stations, or areas of industrial customers that have an exceptionally poor noise spectrum. For example, an attenuation element for the frequency range of data transmission can be installed between the line terminal in the distributor box and a tapping point in an unspliced section of the low-voltage cable. A separate physical coupler that is connected to the repeater unit or passively connected to the next physical coupler is used for each line terminal because, as a result of the attenuation element installed, the data signals cannot be switched to another line terminal of another low-voltage cable by direct cable coupling or electromagnetic coupling based on the released signal power.
Other features and useful developments and advantages of the invention are described in the subclaims and in the embodiment described below.